1. Field of Invention
The present invention relates to circuit structure of an audio power amplifier, more specifically, relates to a structure of a circuit of a switching audio power amplifier with de-noise function.
2. Description of Related Art
Since the evolution of the amplifier from a vacuum tube to a transistor, noise interference has always been the biggest problem that audio power amplifier has ever faced. At present, the efficiency of the commonly used Class D Audio Power Amplifier (called class D amplifier hereinafter) is two to five times higher than the Class AB Audio Power Amplifier. Because of its higher efficiency, class D amplifier only requires less power and can reduce the power consumption of a whole system, therefore the cost, size and weight of a system can be significantly reduced.
Class D amplifier converts audio signals into high frequency pulses and switches and outputs the same according to the audio input signals. Some of class D amplifiers use pulse width modulator to generate constant pulses whose width varies with the amplitude of audio signals. Pulses with changing width switch output transistor at a fix frequency.
Theoretically, a class D amplifier would not output sound when there is no signal input. However, in fact class D amplifier still may amplify the previous step signals. If the noise floor of the previous step IC (Integrated Circuit) is too large, class D amplifier may consider the noise as signals to amplify and output to a speaker, therefore human ears may hear the noise of very low energy. Therefore the energy of the noise floor has to be blocked off with circuit design to improve the SNR (signal to noise ratio) and noise floor performance of a system.
The source of noise can be classified into external noise and noise floor of IC itself, the former for example is power supply hum. This is because the class D amplifier uses transistor switch to drain large current from power supply end, but fails to effectively filter the power supply end. Therefore, when the gate signal of the transistor switch is triggered to demand for a pure clean large current to pass through the power supply end, the large current itself contains noise signals instead of pure clean current, therefore the noise may be fed back to the system itself through a feedback circuit.
Another common external noise interference is RFI (radio frequency interference). When RF (radio frequency) signals are close to an audio amplifier, the common-mode configuration of the transistor switch may receive RF signals as well and becomes the noise interference of the audio amplifier itself due to antenna effect. For example the Hum noise when a cell phone is close to a speaker.
A common practice of eliminating external noise is to apply a low pass filter on the input end to filter the noise mixed in the input signals, so that a load (for example a speaker) does not output noise due to noise signal. Or add a low pass filter circuit in a feedback circuit to eliminate power supply hum noise and to avoid noise being feed back from the feedback circuit. Common-mode input can also be used to drive the load, so as to use high common-mode rejection ratio to counteract the RF interference. Or add a relatively low noise input step in previous step, and then reduce the noise which is output from the present step using a feedback method.
The noise floor of IC itself is an internal spontaneous phenomenon which includes flicker noise, thermal noise and shot noise and etc. The flicker noise is a result of that during the fabrication process of CMOS, the stray capacitor captures electrons and releases electrons with an energy levels method. Thermal noise is a thermal fluctuation phenomenon, which occurs when the IC internal electrons flow through a current pass. Shot noise is the shooting noise (like the whistle sound of BBs of BB gun), which occurs when electrons pass through transistor potential steps. All these are parameters having been determined during the fabricating process of silicon wafers. Therefore, even if there is no audio signal input, but the input end of class D amplifier receives the noise floor of previous step and amplifies it, so very low energy noise at a load end (for example a speaker) can be heard by human ears. Therefore, when there is no audio signal input, after the noise floor received by class D amplifier is amplified, the performances of SNR and noise floor get worse. FIG. 1 shows a known class D amplifier. The input audio signals are input into an input amplifier 101 through capacitors CIN1 and CIN2. Then the comparator 105 compares the amplifier 101's output value received at the negative input end and a triangle wave generator 103's triangle wave reference signals received at the positive end, so as to generate PWM (pulse width modulation) signals. The PWM signals are input to a gate driver 107 to drive the transistors 11˜Q14. The differential outputs OUTP and OUTN of class D amplifier pass through a low pass filter (including L1/C1 and L2/C2) respectively, the latter converts the pulses back to audio amplifying signals to drive a load or a plurality of loads 109 (for example audio speaker).
The differential output OUTP and OUTN of known class D amplifier are complementary, the signal shift range is from ground to VDD. FIG. 2A˜FIG. 2C respectively show the wave forms of large input signals, small input signals and the wave form when no signal is input. In FIG. 2C, a high output ripple current is generated. As shown in FIG. 1, a large LC filter is required in order to reduce the rippled current. In the existing technology, since LC filter is required to eliminate noise signals, therefore the size of the whole circuit may be relatively large.